1. Field of the Invention
This invention relates to a digital video display system and more particularly to such a display system with control means to provide slow or smooth scrolling for a more natural display of characters.
2. Description of the Prior Art
The prior art video character displays or video terminals display characters which are formed of a matrix of discrete points and the control of the display is provided by a character storage in which are stored the signals to be displayed. There is normally a one-to-one relation between storage locations in the control storage and the character space to be displayed during the raster scan of the display screen. The control of such prior art displays is normally provided by a scan line counter and a position counter which are employed to address the control storage in synchronism with the raster scan. One of the problems encountered in such prior art digital video displays is the inability for slowly moving the display characters up or down in a slow or smooth manner to give the appearance of slow scrolling. When the character lines in the prior art systems are to be moved, it requires that the control storage be reloaded with the proper sequence of signals. Another method of providing scrolling in prior art devices is to change the sequence of character codes addressing the control storage. Such prior art digital video displays give the appearance of erasing the message being displayed and regenerating it starting at the top of the display. Other methods give the appearance of individual character lines being moved up in discrete steps, one line at a time, which becomes annoying to the viewer. Such prior art video displays are disclosed for example in the Cole U.S. Pat. No. 3,345,458.
Commercial digital video systems are without exception incompatible with broadcast video standards and use a standard television set or monitor in such a crude fashion that half of its spatial and all of its gray-scale resolution is lost in forming an image.
The most important reason why designers have avoided the standard broadcase type of video raster is that it is interlaced. By producing the complete frame with two interlaced fields, broadcast video achieves twice the vertical resolution that can be achieved at that scan rate without interlace. However, dot-matrix characters appear to "flicker" when interlaced fields are used, particularly if the great majority of dots over any local region happen to fall within one or the other field. Flicker is a problem when interlace is employed not only for dot-matrix characters but also when there are very high contrast images. Thus, even when a standard video monitor is employed as the display device for a video terminal, it is used without interlaced and is limited in its vertical resolution. Because this resolution is inadequate to display a large number of good quality dot-matrix characters, some systems must employ video rasters which are incompatible with standard video monitors. It is desirable to employ standard video monitors because of their economy due to mass production.
It is then an object of the present invention to provide a video display system which provides a natural display of characters and other information.
It is another object of the present invention to provide a digital display system having interlaced scan which nevertheless allows for displayed character lines to appear to move up or down slowly in a scrolling manner.
It is still another object of the present invention to provide a video digital display system that can take advantage of standard commercial video monitors.